No less than thirty-three MacTiernans are mentioned in the "Annals of the Four Masters", practically all of them Chiefs of Teallach Donnchadha (modern Tullyhunco, in the county of Cavan) or their relatives. Though not much information is given about their exploits, the mere recording of so many obituaries indicates importance of the sept throughout the three centuries from 1250-1550. The name is still found chiefly in the Cavan-Leitrim area but generally without the prefix Mac; when the Mac is retained MacTernan is now the usual form. Another sept of MacTiernan held territory in the north-eastern part of Co. Roscommon in mediaeval times. Their origin is different from the Tullyhunco sept, being descended from Tiernan, grandson of Turlough Mor O'Connor, King of Ireland, while the Cavan sept is a branch of the O'Rourkes. This being so, it is of interest to note that the large estate of Hugh MacTernan of Heapstown House, Boyle, Co. Roscommon, in 1878 lay for the most part in Co. Leitrim.

The name in Irish is Mac Tighernain (derived from tigherna, a lord). It is also spelt with the T aspirated - MacThighearnain - which was phonetically anglicized MacKiernan. The Chiefs of Tullyhunco were occasionally called MacKiernan instead of MacTiernan. To-day the two names, including their variants with and without the Mac, are about equal in numbers, and Kiernan is numerous in the same area as Tiernan. The latter is sometimes confused with Tierney, but there is actually no connection between them.

In his Ordnance Survey letters (Co.Cavan) O'Donovan gives a pedigree of this family. In modern times the two best known people of these names are associated with America: Frances Christine Fisher Tiernan (1846-1920), wife of James Tiernan, Catholic novelist and devoted supporter of the Confederates in the Civil War; and Francis Kernan (1816-1892), Democratic senator (Kernan is also a synonym of Kernaghan). At home Most Rev. Edward Kernan (d. 1844) was Bishop of Clogher for over twenty years.